The top seeds are in the two top places as GM Hikaru Nakamura at 4-1 leads GM Wesley So by a half-point through Sunday’s Round 5 of the U.S. Open Championships, now underway at the Chess Club and Scholastic Center of St. Louis.

Nakamura and So, both in the world’s top 10, drew a fascinating game in their much-anticipated individual encounter, but So was upset by 13-year-old Samuel Sevian, the youngest grandmaster in U.S. history playing in his first national title tournament.

On the women’s side, favorite GM Irina Krush also was upset, losing to University of Maryland/Baltimore County star IM Nazi Paikidze in Round 3. But Krush remains very much in the hunt for what would be her seventh U.S. women’s title at 3-1, alone in second a half-point behind Texas WGM Katrina Nemcova.

Easily the most startling result from St. Louis so far was Sevian’s victory over So, ranked No. 5 in the world. Sevian actually walks into some slick opening preparation from White (13. Bxh7+ — Aronian lost in spectacular fashion after 13. Nxh7 — Kh8 14. f4!) in this highly topical QGD Semi-Slav opening, and quickly finds himself on the ropes. But when his higher-rated opponent fails to land the knockout blow, Sevian claws his way back into the game and launches a winning attack of his own.

So blitzed out the first 16 moves, armed with an improvement to the famous Aronian-Anand 2013 game that produced a spectacular victory for Black. Sevian’s position comes close to collapse after 16. Nxb5 Qe7?! (Bb8 right away was better), when White could have played 17. Nxd6! Qxd6 18. Qxg6 cxd4 19. Qh6+ Kg8 20. e4 Qe7 21. Qg6+ Kh8 22. Rf3, with a ferocious attack.

But Black preserves his bishop after 17. Qxg6?! Bb8, and Sevian proceeds to put up a stout defense in the wickedly complex position that results. Black finally turns the tables after 22. b4?! Rcxg5! (removing a key attacking piece and clearing the way for Black to launch a counterstrike of its own) 23. fxg5 Be5 24. Bb2?! (the postmortem determined that the bizarre 24. Nb3! was actually best, as 24…Bxa1 25. Nxa1 leaves Black dangerously exposed on the long diagonal) Ng4 25. Qh5? (a clear mistake — 25. Qh3 was mandatory) Qxg5! 26. Qh3 Qe3+! 27. Qxe3 Nxe3, and Black’s three minor pieces for the rook prove a winning edge.

So gets a pawn to the seventh rank, but can’t stave off the cluster of rooks, bishops and knights surrounding his king. After 40. h6 Rf7 41. h7 Rf1+, White resigned as his king will soon be mated.

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An unlikely final pairing produced an unexpected winner, but Ukrainian IM Mariya Muzychuk nevertheless earned her victory in the just-completed FIDE Women’s World Championship knockout tournament in Sochi, Russia. Muzychuk won the only decisive game of the four-round final against Russian WGM Natalia Pogonina to become the 15th official women’s world champion.

Muzychuk’s “reign” may be short-lived — she now must play a title match against the current queen of the women’s game, GM Hou Yifan of China, who was not in the field in Sochi.

Muzychuk, who eliminated top-seeded Indian GM Humpy Koneru on her way to the winner’s circle, will be a decided underdog against Hou, but her impressive play in Sochi suggests she shouldn’t be underestimated. Her crucial win over Pogonina was not without flaws, but she established a clear bind in an opening she rarely plays — a Breyer Ruy Lopez — and never gave Black a clear shot to get back into the game.

Savielly Tartakover once remarked that playing the White side of the Ruy is like “milking a cow,” and it’s Black’s job to somehow kick over the milk pail. Pogonina never quite manages that here (31…Qf4, instead of the game’s 31…Qe7 32. Qf4 Nfd7, might have provided more counterplay), and finds her queenside blocked and her bishop on b7 permanently locked out of play. White methodically builds up her attacking array on the center and kingside.

White misses a few quicker routes to victory (the computer suggests the startling 46. Ndf5!! as one crusher, offering 46…Nh7 [gxh5 46. R1xe5 Qe8 47. Rh6] 46. Nh6+ Kg7 47. R6f5! gxf5 48. Rxf5 Qe8 49. Qd4+ f6 50. gxf6+ Kxh6 51. e5 dxe5 52. Rxe5, and the Black king can’t survive), but with Muzychuk’s domination of the half-open f-file, the d4-square available to her knight, and her active queen, the breakthrough soon becomes just a matter of time.